As semiconductor and integrated circuit technology has advanced, there has been a trend toward high-functionality integrated circuit components with numerous input and output (I/O) pads, together with a demand for reduced chip size, weight, and power consumption. Consequently, as integrated circuits get smaller, they increasingly have smaller I/O pads arranged more closely together than ever before.
To match these high functionality integrated circuits, there is a demand for printed wiring boards having closely arranged pads for integrated circuit component attachment. To date, the ability to fabricate substrates with sufficiently fine pitch component attach pads has not been able to keep up with miniaturization in integrated circuit components. Consequently, there is an interconnection technology gap for some modern devices.
To make such devices function, printed wiring boards may have extra routing layers to handle the pads of the integrated circuits, or utilize fan-out packaging. This results in the package size of an integrated circuit being larger than the integrated circuit itself, which may limit system miniaturization. In addition to these desires for miniaturized devices, it is also desirable in some cases to construct these devices from a flexible, and not rigid, substrate.
One material now being used as a substrate from which to construct thin and flexible printed wiring boards is biaxially-oriented liquid crystal polymer (LCP). The molecules in LCPs have rigid, rod-like shapes, and maintain a crystalline order when in a liquid phase or when heated and melted. The Processing and Assembly of Liquid Crystalline Polymer Printed Circuits, T. Zhang, W. Johnson, B. Farrell, and M. St. Lawrence, “The processing and assembly of liquid crystalline polymer printed circuits,” 2002 Int. Symp. on Microelectronics, 2002. discusses the construction of a printed circuit board using LCP as a substrate. A photoresist is first applied to a copper clad laminate, exposed, and developed to define a desired circuit pattern. The actual circuit is defined by etch removal of any exposed copper. Holes or vias are created in the substrate via mechanical or laser drilling. A desmearing step is performed to remove debris from the vias or holes, thereby preparing the LCP material for metal deposition. A metalization step is next performed, and a conventional solder mask is applied to the LCP substrate. Solder is then applied through the conventional solder mask to complete the construction of the LCP printed circuit board.
While this design does allow for the creation of thin, flexible printed circuit boards, it still suffers from the same drawbacks as described above with respect to the attachment of integrated circuits with closely spaced pads.
Traditional semiconductor processing techniques allow for fabrication of rigid, wafer substrates that would support the levels of component attached referenced above. In this process, metals are deposited from the vapor phase into very thin films and are lithographically patterned and etched similar to that described for printed circuit boards. Dielectric layers are formed between metal layers through a spun on or vapor phase deposition process. While allowing for attachment of fine pitch components, this approach lacks the ability to achieve flexible circuits due to the rigid wafer substrate requirement for semiconductor processing. As such, additional methods of connecting integrated circuits to flexible printed circuit boards are needed.